1. Field of the Invention
The present invention relates to making, in monolithic form, bidirectional switches of medium power.
2. Discussion of the Related Art
Conventional static bidirectional switches are triacs. A triac corresponds to the antiparallel association of two thyristors. It can thus be directly connected in an A.C. network, for example, the mains. The gate of a conventional triac corresponds to the cathode gate of one, at least, of the two thyristors forming it and is referenced on the electrode located on the front surface of this triac, that is, the surface that includes the gate terminal, while the other triac surface is typically connected to a heat sink or to the ground.
Bidirectional switches of the type described in European patent application No. 0 817 277, the triggering of which is ensured by applying a voltage between a control electrode located on the front surface of the component and a main electrode located on the opposite surface of the component, will more specifically be considered hereafter.
FIG. 1 shows an equivalent schematic diagram of such a bidirectional switch. A control electrode G of the bidirectional switch is connected to the emitter of a bipolar transistor T, the collector of which is connected to the anode gates of first and second thyristors Th1 and Th2 placed in antiparallel between two terminals A1 and A2. Terminal A1 corresponds to the anode of thyristor Th1 and to the cathode of thyristor Th2. Terminal A1 is also connected to the base of transistor T. Terminal A2 corresponds to the anode of thyristor Th2 and to the cathode of thyristor Th1.
FIG. 2A is a simplified cross-section view of an example of a monolithic embodiment of the bidirectional switch described in relation with FIG. 1. Transistor T is formed in the left-hand portion of the drawing, thyristor Th1 at the center, and thyristor Th2 to the right thereof. As will be seen hereafter, this does not correspond to the effective arrangement of the various components, an example of which will be given in relation with the top view of FIG. 2B, but is only intended for explaining the operation of the structure.
The structure of FIG. 2A is formed from an N-type lightly doped semiconductor substrate 1. The anode of thyristor Th1 corresponds to a P-type layer 2 that is formed on the rear surface side of substrate 1. Its cathode corresponds to an N-type region 3 formed on the front surface side in a P-type well 4. The anode of thyristor Th2 corresponds to a P-type well 5 formed on the front surface side and its cathode corresponds to an N-type region 6 formed on the rear surface side in layer 2. The periphery of the structure is formed of a heavily-doped P-type layer 7 extending from the front surface to P-type layer 2. Conventionally, region 7 is obtained by drive-in from the two substrate surfaces. The rear surface is coated with a metallization M1 corresponding to first terminal A1 of the bidirectional switch. The upper surfaces of regions 3 and 5 are coated with a second metallization M2 corresponding to second terminal A2 of the bidirectional switch. An N-type region 8 is formed, on the front surface side, in a P-type well 9 in contact with peripheral region 7. The surface of region 8 is contacted by a metallization M3 connected to control terminal G of the bidirectional switch. A metallization M4 may be formed on the upper surface of peripheral region 7. Metallization M4 is connected to no external terminal. As an alternative, well 9 may be separated from peripheral region 7 and electrically connected thereto via metallization M4.
The operation of this bidirectional switch is the following.
When terminal A2 is negative with respect to terminal A1, thyristor Th1 is capable of being on. If a sufficiently negative voltage with respect to metallization M1 is applied to gate G, the base-emitter junction of transistor T is forward biased and this transistor turns on. A vertical current ic shown in dotted lines in FIG. 2A thus flows from metallization M1, through the forward junction between layer 2 and substrate 1, then into regions 1, 9 and 8 corresponding to transistor T. Carriers are thus generated at the level of the junction between substrate 1 and well 9 near the junction between substrate 1 and well 4, and thyristor Th1 is turned on. It can also be considered that an auxiliary vertical NPNP thyristor Tha including regions 8-9-1-2, region 9 of which forms the cathode gate region, has been triggered.
When terminal A2 is positive with respect to terminal A1, thyristor Th2 is capable of being on. Applying a negative voltage on terminal G turns on transistor T. The carriers present in the vicinity of the junction between substrate 1 and layer 2 turn on thyristor Th2, as will be better understood by referring to the simplified top view of FIG. 2B in which it can be seen that the region corresponding to transistor T neighbors a portion of each of thyristors Th1 and Th2.
As can also be seen in the top view of FIG. 2B, triggering transistor T (that forms a portion of auxiliary thyristor Tha) is arranged to face a portion of vertical thyristor Th1 and a portion of vertical thyristor Th2. More specifically, thyristor Th1 must be very sensitive in the vicinity of its triggering area, that is, it must include no short-circuit region between its cathode and its cathode gate. Thus, metallization M2, not shown in FIG. 2B, which is in contact with region 3 and well 5, is in contact with region 4 in a region opposite to the triggering area only.
A disadvantage of the structure shown in FIG. 2 is that it has a poor switching breakdown voltage characteristic in the presence of an inductive load, that is, when the conduction of the bidirectional switch is desired to be interrupted, said switch, and more specifically thyristor Th1, risks turning back on while its control electrode is no longer activated.
An object of the present invention is to provide a novel embodiment, in monolithic form, of a bidirectional switch of the above mentioned type that has a better switching breakdown voltage, especially on an inductive load.
To achieve this and other objects, the present invention provides a monolithic bidirectional switch formed in a semiconductor substrate of a first conductivity type having a front surface and a rear surface, including a first main vertical thyristor, the rear surface layer of which is of the second conductivity type, a second main vertical thyristor, the rear surface layer of which is of the first conductivity type, and structures for triggering each of the first and second main thyristors arranged to face regions mutually distant from the two main thyristors, the neighboring portions of which correspond to a region for which, for the first main thyristor, a short-circuit area between cathode and cathode gate is formed.
According to an embodiment of the present invention, the bidirectional switch includes a first auxiliary vertical thyristor, the rear surface layer of which is of the second conductivity type and is common with that of the first main thyristor, a second auxiliary vertical thyristor, the rear surface layer of which is of the second conductivity type and is common with that of the first thyristor, the main upper surface terminals of the first and second auxiliary thyristors forming a same control terminal, a peripheral region of the second conductivity type connecting, in particular, the rear surface layer of the auxiliary thyristors to the gate layers of these auxiliary thyristors located on the other side of the substrate, a first rear surface metallization, a second front surface metallization connecting the front surface regions of the first and second thyristors.
According to an embodiment of the present invention, the bidirectional switch includes an additional region that isolates the rear surface of the first auxiliary thyristor from the first metallization.
According to an embodiment of the present invention, the bidirectional switch includes, in the substrate between the upper surface of each of the main thyristors and the upper surface of each of the auxiliary thyristors, a region of the first conductivity type more heavily doped than the substrate, connected to the other region.
The foregoing objects, features and advantages of the present invention, will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings.